1. Field of the Invention
The present invention relates to a spring steel for use in the production of springs, and more particularly to a spring steel having a remarkably low decarburizability during the hot rolling thereof (rolling of a bar stock or a plate material) as well as during heating and heat treatment thereof for hot forming and working thereof.
2. Description of the Prior Art
In general, flat springs, coil springs and torsion bars are produced through quench hardening and heat treatment such as tempering of a hot-rolled material. If the surface of a steel material is decarburized in the foregoing production process, the fatigue strength thereof is notably lowered to present a problem that the resulting product cannot exhibit the properties of a spring.
Accordingly, a method wherein a coating layer of a decarburization-proofing agent is formed on the surface of a steel material, followed by heat treatment thereof, a method wherein a steel material is heat-treated in an atmosphere, and the like have heretofore been adopted as decarburization-preventive technologies.
A method wherein a variety of element(s) is added to a steel material to lower the decarburizability thereof has also been adopted. For example, Japanese Patent Laid-Open No. 170,542/1986 discloses Cu and Ni as such additive elements, Japanese Patent Laid-Open No. 274,058/1987 discloses Sb as such an additive element, and Japanese Patent Laid Open No. 223,148/1988 discloses Pb, Bi and Sn as such additive elements.
The conventional decarburization-preventive technologies involve the following problems, because of which an increase in cost cannot be avoided.
a. The method wherein a coating layer of a decarburization-proofing agent is formed on the surface of a steel material is time-consuming because of the time for application and it requires equipment therefor.
b. The method wherein a steel material is heat-treated in an atmosphere is costly in connection with the formation of an atmospheric gas and it involves the use of an atmospheric heating furnace, which is expensive as compared with a common furnace.